A variety of techniques are available for providing visual displays of graphical or video images and for acquiring images. In many display applications, cathode ray tube type displays (CRTs), such as televisions and computer monitors produce images for viewing. Such devices suffer from several limitations. For example, CRTs are bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications.
Matrix addressable displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power. However, typical matrix addressable displays utilize screens that are several inches across. Such screens have limited use in head mounted applications or in applications where the display is intended to occupy only a small portion of a user's field of view. Such displays have been reduced in size, at the cost of increasingly difficult processing and limited resolution or brightness. Also, improving resolution of such displays typically requires a significant increase in complexity.
Devices for acquiring images, such as cameras and scanning microscopes may have similar constraints. For example, small cameras often use CCD arrays to convert light energy to electrical signals. In high resolutions systems, the CCD array can be complex. Full color CCD applications can add further complexity.
One approach to overcoming many limitations of conventional displays or image capture devices is a scanned beam approach, such as that described for displays in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference or as described for image capture in U.S. Pat. No. 5,742,419 to Dickensheets et al., entitled MINIATURE SCANNING CONFOCAL MICROSCOPE, which is incorporated herein by reference.
As shown diagrammatically in FIG. 1, in one embodiment of a scanned beam display 40, a scanning source 42 outputs a scanned beam of light that is coupled to a viewer's eye 44 by a beam combiner 46. In some scanned displays, the scanning source 42 includes a scanner, such as scanning mirror or acousto-optic scanner, that scans a modulated light beam onto a viewer's retina. In other embodiments, the scanning source may include one or more light emitters that are rotated through an angular sweep.
The scanned light enters the eye 44 through the viewer's pupil 48 and is imaged onto the retina 59 by the cornea. In response to the scanned light the viewer perceives an image. In another embodiment, the scanned source 42 scans the modulated light beam onto a screen that the viewer observes. One example of such a scanner suitable for either type of display is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference.
Sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user's field of view and presents an image that occupies a region 43 of the user's field of view 45, as shown in FIG. 2A. The user can thus see both a displayed virtual image 47 and background information 49. If the background light is occluded, the viewer perceives only the virtual image 47, as shown in FIG. 2B.
One difficulty that may arise with such displays is raster pinch, as will now be explained with reference to FIGS. 3-5. As shown diagrammatically in FIG. 3, the scanning source 42 includes an optical source 50 that emits a beam 52 of modulated light. In this embodiment, the optical source 50 is an optical fiber that is driven by one or more light emitters, such as laser diodes (not shown). A lens 53 gathers and focuses the beam 52 so that the beam 52 strikes a turning mirror 54 and is directed toward a horizontal scanner 56. The horizontal scanner 56 is a mechanically resonant scanner that scans the beam 52 periodically in a sinusoidal fashion. The horizontally scanned beam then travels to a vertical scanner 58 that scans periodically to sweep the horizontally scanned beam vertically. For each angle of the beam 52 from the scanners 58, an exit pupil expander 62 converts the beam 52 into a set of beams 63. Eye coupling optics 60 collect the beams 63 and form a set of exit pupils 65. The exit pupils 65 together act as an expanded exit pupil for viewing by a viewer's eye 64. One such expander is described in U.S. Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporated herein by reference.
One skilled in the art will recognize that, for differing applications, the exit pupil expander 62 may be omitted, may be replaced or supplemented by an eye tracking system, or may have a variety of structures, including diffractive or refractive designs. For example, the exit pupil expander 62 may be a planar or curved structure and may create any number or pattern of output beams in a variety of patterns. Also, although only three exit pupils are shown in FIG. 3, the number of pupils may be almost any number. For example, in some applications a 15 by 15 array may be suitable.
Returning to the description of scanning, as the beam scans through each successive location in the beam expander 62, the beam color and intensity is modulated in a fashion to be described below to form a respective pixel of an image. By properly controlling the color and intensity of the beam for each pixel location, the display 40 can produce the desired image.
As the beam is scanned, a variety of optical effects may affect the quality of the displayed or captured image. For example, millimeter scale optical instruments typically require lens elements in the intermediate size range of several hundred micrometer clear apertures. Although considerable progress has been made in the fabrication of single refractive microlenses in this regime, the construction of high-power, high-resolution systems of refractive microlenses capable of diffraction limited imagery over a large field of view remains a considerable technological challenge.
When attempting to form an image with a simple lens (one that is not fully corrected), one observes aberrations in the image that depend on the location in the field-of-view. The optical surface that works optimally for resolving a point in the center of the field is usually not quite the same as the optical surface that optimally resolves a point off to one side in the field-of-view. To solve this problem, the lens designer typically adds additional lens surfaces until the system is fully corrected throughout the field-of-view. This can add considerable complexity for the micro-optical instrument, and in many cases a practical design may not be realized. If, however, the design constraints are reduced so that the lens design can be optimized for one particular position in the image field, a much simpler lens could be used; however, the optimal lens would usually be slightly different for each pixel. Array based imaging systems that image all of the pixels in the field simultaneously therefore use lens systems fully corrected to the desired level of performance.
Considerable effort has been devoted to developing the theory and devices for adaptive optics for the correction and optimization of imaging systems such as telescopes and antennas. Much of that work has focused on random aberrations from such external sources as atmospheric turbulence, and the surfaces required to compensate such fluctuations are highly variable. Adaptive mirrors for these applications typically have tens or hundreds of actuators in an array pattern, often with a thin continuous membrane over the surface of the array. Control of such a surface often uses local wavefront sensing and feedback control, and these surfaces typically are for slowly varying aberrations.
In some examples of such an approach continuous membrane micromachined adaptive mirrors are use for correction. Such approaches are described in Gleb Vdovin and P. M. Sarro, “Flexible mirror micromachined in silicon,” Applied Optics vol. 34 no. 16, pp 2968-2972, 1995; P.K.C. Wang and F. Y. Hadeagh, “Computation of static shapes and voltages for micromachined deformable mirrors with nonlinear electrostatic actuators,” JMEMS vol. 5 no. 5, pp 205-220, 1996; Thomas G. Bifano et. al., “Continuous-membrane surface micromachined silicon deformable mirror,” Optical Engineering, vol. 36 no. 5, pp 1354-1359, 1997; Linda M. Miller, Michael L. Agronin, Randall K. Bartman, William J. Kaiser, Thomas W. Kenny, Robert L. Norton and Erika C. Vote, “Fabrication and characterization of a micromachined deformable mirror for adaptive optics applications,” SPIE 1945, 1993; and Adrian M. Michalicek, Natalie Clark, John H. Comtois, Heather K. Schriner, “Design and simulation of advanced surface micromachined micromirror devices for telescope adaptive optics applications,” SPIE 3353, pp. 805-815, 1998 each of which is incorporated herein by reference.
Returning to the general description of scanning, simplified versions of the respective waveforms of the vertical and horizontal scanners are shown in FIG. 4. In the plane 66 (FIG. 3), the beam traces the pattern 68 shown in FIG. 5. Though FIG. 5 shows only eleven lines of image, one skilled in the art will recognize that the number of lines in an actual display will typically be much larger than eleven. As can be seen by comparing the actual scan pattern 68 to a desired raster scan pattern 69, the actual scanned beam 68 is “pinched” at the outer edges of the beam expander 62. That is, in successive forward and reverse sweeps of the beam, the pixels near the edge of the scan pattern are unevenly spaced. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because the image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern 69, the displaced pixel locations can cause image distortion.
Further optical aberrations in the optical train may produce pixel non-uniformities, distortion, or other artifacts that may reduce the appearance or degrade the performance of a scanned display or image capture device.